Solid electrolytic capacitors and, in particular, tantalum capacitors, are used to store energy. Tantalum capacitors are used in various applications because of their superior volumetric efficiency, one such application being pulsed radar systems. In such a system, high capacitance and low equivalent series resistance (ESR) allow for maximization of energy transfer during the “pulse” of the radar system.
Radar system designs incorporate many tantalum capacitors, which are electrically placed in parallel to achieve the required system capacitance value. Fuses are often placed in series with each capacitor in a radar system to prevent the capacitor from igniting upon exposure to excess current by disconnecting the capacitor from the circuit. If a tantalum capacitor fails due to an electrical short, the capacitor may ignite and destroy the printed circuit board that the capacitors are mounted on. This risk is particular to a radar system that has a large number of capacitors mounted in parallel.
In order to meet miniaturization requirements, some circuit designers have requested that tantalum capacitor manufacturers put a fuse into the capacitor in order to design out the discrete fuse. Tantalum capacitor manufacturers have done this to some extent, by incorporating fuse assemblies with the capacitor package. Generally, two packages are available.
The first package is a fused molded solid electrolyte tantalum capacitor, which has a capacitor body and a fuse assembly encapsulated in a plastic outer shell. The implementation of a fuse assembly requires an increase in the overall package size of approximately 35%. In a typical molded chip, the tantalum capacitor body is about 35% of the total volume of the molded package. Addition of the fuse assembly further reduces the percentage of capacitor body relative to package size, thus negatively affecting the advantage of volumetric efficiency. Furthermore, the fuse assembly is not integrated into the capacitor body and, therefore, a considerable ESR increase is realized because of the added connections and the length of those connections. In fact, the resistance of the fuse and its connections can add 100 mOhms of resistance to the finished capacitor.
The second package is a conformal-coated fused capacitor. In this package, the capacitor body is coated with a conformal material rather than with the plastic packaging of the molded device. The volumetric efficiency of the conformal-coated tantalum capacitor is much higher than that of the molded package because the capacitor body is 75% of the total package. If, however, a fuse assembly is added to the capacitor body, then it is added on the outside surface of the capacitor body and increases the overall package size. This method adds considerable ESR to the capacitor due to the necessary connections and may cause circuit assembly pick and place problems because of the protruding fuse.
What is needed therefore is a fused tantalum capacitor that integrates the fuse within the capacitor body so as to not affect the package size and to minimize the increase in ESR.